Different imaging techniques have been developed for monitoring and control purposes.
For example, because of their extended area inspection and their small wavelength, which better interacts with small defects, Structural Health Monitoring (SHM) techniques based on guided wave propagation in structures have been used for many years.
The reflection of guided waves with defects in composite and metallic structures may be used for localizing such defects.
Mode conversion at defects has been exploited in detection strategies, either for simple structures using A0 and S0 Lamb waves, or in plate overlaps using incident A0 and S0 Lamb waves, and shear horizontal (SH) waves.
Efficient sensing and actuating schemes for SHM have been demonstrated using multiple piezoelectric elements. For example, approaches for embedded damage detection using pitch-catch configurations where piezoelectric elements are used on both sides of a suspected damage area for simple and complex structures have been disclosed. The representation of the energy carried by a propagating wave has been used in many approaches to identify reflection and transmission at discontinuities.
In order to minimize the footprint of sensors required for embedded damage detection, compact sensing strategies have been disclosed for pulse-echo configurations, with various array configurations.
Most of the damage detection and localization approaches are currently based on the measurement of a time-of-flight (ToF) and the knowledge of the group velocity for a mode propagating at a given frequency. Such approaches have been used within imaging techniques to process the signals measured by the elements of arrays.
As an example, the Embedded Ultrasonic Structural Radar (EUSR) uses a phased-array approach with a round-robin procedure to image defects located, in its simplest implementation, in the far-field of the array. In these approaches, the localization of the reflectors in the image relies on the maximum of the envelope of the measured burst.
For non-dispersive propagation, an accurate localization can be obtained. However, even if specific low-dispersive modes are injected and/or measured in the structure using selective actuators and sensors, mode conversion at discontinuities might generate dispersive modes which may superimpose with the targeted modes and significantly complicate the measurement of the ToF associated with the echoes in the time domain signal and therefore impair the localization of the reflectors and lead to biased diagnostic.
A number of approaches have been proposed to extract mode-related information from a time domain signal. The matching pursuit approach has been proposed to decompose time domain signals using Gabor time-frequency atoms. The approach has been improved using Gaussian chirps, trying to mimic the excitation signal used for detection, or using shifted and scaled versions of the excitation signal.
The analysis of dispersion using time-frequency tools has also attracted much attention, with new transforms such as chirplet transform and more recently, the warpogram. A number of researchers have proposed ways to compensate for the effect of the dispersion so that the shape of the input signal can be recovered in the measured signal, and thus a better estimate of the ToF may be provided. The comparison of various approaches has shown that dispersion compensation provides the least error on the estimate of the ToF.
However, these techniques are often quite complicated to implement and may lead to approximate localization of the defects present in the structure under inspection, which is a great disadvantage.
It would therefore be desirable to provide an improved method for imaging a structural condition of a structure that would reduce at least one of the above-mentioned drawbacks.